The present invention relates to the field of linear motors, and in particular to linear motors having a primary and a covered secondary.
In general, a linear motor includes a, normally fixed, secondary or reactive part, having in case of a synchronous linear motor a plurality of magnetic elements (permanent magnets) arranged side-by-side to define a guideway for a rotor which is movable relative thereto and represents the primary part of the linear motor. The rotor interacts with the magnets of the secondary and converts this interaction into a translatory motion.
Such linear motors are used as drives in various technological fields, for example, in machine tools, machining devices such as automatic drills, automatic milling cutters, automatic lathes, automatic placement machines or automatic bonding machines, including vehicle drives in magnetic levitation trains, etc.
The structural length of the guideway depends on the intended use of the linear motor and should be at least as long as the required displacement path plus the length of the rotor. In order to have flexibility in connection with the design of linear motors of different length, the secondary is made up of single short secondary members, which are arranged side-by-side until reaching the desired length.
The hereby created butt-joints are, however, disadvantageous in particular when exposed to rough whether, because dust, operating liquid, aggressive materials etc. can accumulate there. It also happens frequently that solid structures, tool pieces etc. migrate onto the guideway, for example, when drill bits break off from automatic drills. In the most adverse case, these solid materials may be snatched by the rotor and forced in the area of the butt-joints into the secondary.
To avoid this, the guideway is, typically, provided with a cover which is normally designed as a plastic film glued onto the guideway of the secondary member. Oftentimes, however, the butt-joints cannot be neutralized adequately, thereby causing unevenness, and thus are subjected to the afore-mentioned mechanical stress; However, also other regions can be damaged fairly easily. Thus, the plastic films must be replaced from time to time. As they are glued onto the guideways of the secondary member, replacement is relatively time-consuming and cost-intensive and causes shutdown periods of the machine.
German Pat. No. DE 195 01 938 A1 describes a moving secondary (or rotor) for a linear motor in which the magnetic elements are enclosed by a cover of, preferably, magnetic material. However, as the rotor of the linear motor is concerned here, the afore-stated problems as relating to long stators are not relevant here. Furthermore, the cover is connected with fasteners, namely screws, for the entire length of the armature so that a simple exchange is not possible.
International publication no. WO 00/30240 describes a linear motor having a secondary composed of plural secondary members, wherein the guideway of the secondary is protected by a cover. The cover contains magnetizable material for adherence to the secondary members. In particular when dirt or foreign matters such as chips are encountered between the primary (or rotor) and the secondary, jamming may be experienced so as to expose the cover to a force. The magnetic cover can thus shift during operation of the linear motor or even detach from the secondary as the static friction between cover and secondary is not sufficient.
It would therefore be desirable and advantageous to provide an improved secondary for a linear motor and an improved linear motor to obviate prior art shortcomings and to enhance the attachment of a cover to the secondary.